What Does the IQ Scale Measure?

What does the intelligence quotient scale really measure?
What is an average score? How high can a person's IQ be? When
did we first use this particular measurement of intelligence?
We'll start with that last question.

The imperial examination system in China, established in 605
during the Sui Dynasty, may have been the first large-scale testing
of mental abilities. It was used to choose the best candidates
for administrative positions in the Chinese bureaucracy. In the
19th century testing of mental capacity became common in France.
In 1904 psychologist Alfred Binet was commissioned by the French
government to create a testing system to differentiate intellectually
normal children from those who were mentally deficient.

From Binet's work the IQ scale called the "Binet Scale"
was developed. With psychologists Victor Henri and Théodore
Simon he developed the Binet-Simon test, which was first published
in 1905. The term "Intelligence Quotient," came from
the very similar German term Intelligenz-Quotient (or Intelligenzquotient),
which was first used by psychologist William Stern. The acronym
"IQ" is used most often now. Lewis M. Terman revised
the Simon-Binet IQ Scale, and in 1916 published the Stanford
Revision of the Binet-Simon Scale of Intelligence (also known
as the Stanford-Binet). The following scale resulted for classifying
IQ scores:

Normal Distribution of IQ Scores

Modern tests are designed so that the average (mean) score
is 100. The standard deviation is usually 15, and about 95% of
scores fall within two standard deviations, which means that
most of us have a score between 70 and 130. Here is a more detailed
breakdown of the distribution of scores:

50% of IQ scores fall between 90 and 110
70% of IQ scores fall between 85 and 115
95% of IQ scores fall between 70 and 130
99.5% of IQ scores fall between 60 and 140

Low IQ and Mental Retardation

An IQ under 70 is considered as "mental retardation"
or limited mental ability. 5% of the population falls below 70
on IQ tests. The severity of the mental retardation is commonly
broken into 4 levels:

The Average on an IQ Scale

The scoring of an IQ test is not the same for everyone, because
age is used in determining a score. Intelligence quotient is
determined by the "intelligence age" (IA) in relation
to the chronological age of the person being tested. IQ scales
can differ from each other (Americans use scales with IQ values
above 200), but the mean value of most scales is an IQ of 100.
This represents normal intelligence.

Think you are smart? Well, if your IQ is 130, that puts you
ahead of 98% of people. Of course that means there are still
140 million people who are smarter than you (the other 2%). Also,
recent research shows that if a person is anywhere within the
range of normal intelligence, a person's level of self-discipline
is more predictive of success than their specific intelligence
quotient. In other words, don't take your score on an IQ test
to mean too much.

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